We call this our "90 mm Internal Expanding Rear Brake Drum Assembly" Here is a curious little item, an internal expansion drum brake assembly. Unlike some small scooter brake units that use bands that contract or clamp-down on a spinning drum to provide the braking pressure, this brake unit's shoes expand outwards to grip the inside of the drum. This is essentially the same system used on an automotive drum brake, but on a much smaller scale. External labeling states this is a "Servo Brake" from Xun Lin Brake. This is an outside view the 90 mm unit. There is an almost identical 100 mm version in stock as well.